Bracken Cave, a collapsed sinkhole on the side of a lush limestone hill in southern Texas, is home not only to the world’s largest known bat colony but also to one of the largest concentrations of any mammal. More than fifteen million Mexican free-tailed bats live, for part of the year, deep in its recesses. One evening in late July, I visited the cave, situated just outside San Antonio, with a group of bat-curious tourists. As the sun went down, we watched hundreds of bats emerge every second, fifty yards from where we sat, swirling up in what our guide, Niki Lake, a volunteer from Bat Conservation International, called “a batnado.” They created their own breeze, with its own murmur—an ASMR-inducing summer buzz, insectivorous and thrumming, the sound of thousands of beating wings coming in waves. (Lake called the sound a “bat ocean.”) The show would go on for about four and a half hours, until every last bat that could fly had exited the cave. One member of our party said that, if he had discovered this place in prehistoric times, he would have thought it was a portal to another world. As he spoke, I felt a wet spot on my hand. A bat had peed on me.

The bats are so numerous that they appear on radar systems—great blobs popping up and hovering above Bracken Cave every night. (Traffic controllers at San Antonio International Airport have to direct planes to avoid them.) A study published last year used weather-radar records from south-central Texas to analyze the bats’ migration patterns in the course of more than two decades. “Using radar, we were able to see the bats emerging from Bracken Cave every single night for the past twenty-three years,” Charlotte Wainwright, an atmospheric scientist at Notre Dame, told me. Wainwright and Phil Stepanian, the study’s lead author, found that the bats’ spring migration to Texas—they spend their winters scattered across Mexico—had advanced by two weeks. A small percentage of the bats were not leaving Bracken Cave at all. When researchers first visited, in the late nineteen-fifties, there were no bats present in December and January, and yet, according to the data, there are now fifty thousand or more bats emerging during the winter. In the most extreme cases in recent years, more than a quarter of a million bats are spending at least part of the winter at the cave, Stepanian said. “That’s a very sudden change—to have no animals there, to having some, to having so many.”

On July 23rd, another group of scientists published an article, in the journal Nature Communications, titled, “Adaptive Responses of Animals to Climate Change are Most Likely Insufficient.” There now exists plenty of evidence that animal behavior is changing, adapting to the rapid rate of warming. These adaptations include, most clearly and systematically, as in the case of the Bracken Cave bats, an alteration in the timing of seasonal behaviors like reproduction, hibernation, and migration. But over all, the authors found, these changes won’t be enough to counter the effects of climate change; in some cases, animals’ adaptations might harm them more than help them. The Columbian ground squirrel, for example, has begun emerging from hibernation later, and has been weaker as a result. What this means, of course, is that animals, which are already going extinct at a rate tens to hundreds of times higher than they have in the past ten million years, primarily because of the rapid increase in human population and development, are going to have an even harder time surviving as climate change advances. As a sweeping United Nations report on biodiversity, published in May, concluded, accelerating climate change is a “direct driver” of species extinction and biodiversity destruction, and is “increasingly exacerbating the impact of other drivers.”

Stepanian and Wainwright are now using their radar analysis to track the spectacular emergence of mayflies in the Upper Midwest every summer. Although they haven’t published their findings yet, their data suggests that the number of mayflies has decreased significantly over the past few years. Stepanian is also now using radar to study the migration patterns of Australian bats, specifically gray-headed flying foxes—one of the largest types of bat in the world, with wingspans that can reach three feet. Last November, a heatwave in northern Australia killed one-third, or twenty-three thousand, of the nation’s spectacled flying foxes (also known as spectacled fruit bats) in just two days. Unable to survive the temperatures, which exceeded a hundred and seven degrees Fahrenheit, they fell dead from the sky.

At Bracken Cave, the Mexican free-tailed bats’ shifting migration habits “very likely is due to prey availability, Stepanian told me. The bats survive on insects, which they eat all night as they fly across southern Texas. Insects are very sensitive to temperature, and if it is cold they cannot fly. So hotter winters in Texas mean, in theory, more insects, which means that the bats don’t have to make the long trip south and back to find food in the winter. “A positive spin on this is that the bats are adapting as their environment changes,” Stepanian told me. “The question is, How long can they sustain that adaptation? If less and less are leaving Texas, while more and more are wintering in Texas, we can’t say what that means for disease dynamics. More diseases could spread. There also could be less population mixing.”

A potential negative outcome, Wainwright said, “is the possibility of a mismatch developing between the bats’ migration timing and the arrival of prey species.” One of the bats’ favorite meals is the corn earworm moth, which is a major pest, and, if left untouched, can destroy corn and cotton crops. (According to a 2015 article in Nature Conservancy, the bats save the region’s cotton growers alone seven hundred and forty-one thousand dollars a year in pesticide costs and crop damage.) If the peak arrival of these insects no longer occurs in synchrony with the migration of bats into the region, then there could be cascading effects. Various studies have found, for example, that the entire U.S. bat population provides ecological services to the agricultural industry—including reduced crop loss from insects, reduced spread of crop disease, and reduced need for insecticides—worth as much as twenty billion dollars annually.

In 2013, San Antonio’s urban sprawl threatened the existence of Bracken Cave’s bats. A developer was prepared to build thirty-five hundred tract houses around the cave, directly in the bats’ flight path, a landscape of rolling hills scattered with Texas live oak, persimmon, hackberry, and juniper bush, full of wildlife. While we were sitting watching the cave before sunset, we spotted a Western coachwhip snake, fat and scaly, loafing just above the entrance. (It feeds on grounded bats.) As the bats emerged, hawks shrieked and dived into the swarm to catch dinner. Great horned owls live in the woods, along with an endangered migrating songbird, the golden-cheeked warbler. Scientists and environmentalists were worried that the planned community would have a negative impact on the over-all ecology of the area, and that residents of the new homes would see the swarming bats as pests, and turn against them. (I met two bat-cave neighbors who said that the bats swept into their pool every evening to drink water, but they seemed proud of the fact.) A great fight ensued to stave off the developers and convince state politicians and residents to rally behind the bats. The bats and their benefactors won, preserving the land around the cave for perpetuity. Apart from protecting wildlife, the land also protects an aquifer deep underground—the water source for greater San Antonio.

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At dawn, we returned to the cave with Fran Hutchins, a scientist with Bat Conservation International and the director of the Bracken Cave Preserve. He was one of the leaders of the fight to show people why they should love the bats, and save the land. The adult bats may have flown as far as sixty miles that night, searching for dinner across cropland, he told us, while the juveniles would have had to learn to find food closer to home. To feed their babies, called pups, which are born in early summer, bats will consume as much as two-thirds of their body weight (equivalent to the weight of two quarters in your palm) in one foraging session; all together, the Bracken bats consume about a hundred and forty tons of flying beetles, winged ants, and corn earworm moths (along with other moth species) per night. The pups under five weeks (the age at which they learn to fly) cling to the edges of cave wall, waiting for their nursing mothers, who find them, upon arrival, by their scent and voice. Hutchins said that he may or may not have once sampled bats’ milk. “You have to drink it out of a miniature cup, smaller than a thimble,” he said.

In the dark, through night-vision binoculars, the earliest bats to arrive looked like a flurry of gray-green bow ties, circling the eye-shaped entrance to the cave, as if stuck inside a snowglobe. The limestone cave descends into a deep abyss, where the temperature is at least a hundred and four degrees Fahrenheit. The cave’s floor is coated with bat droppings, or guano (historically mined by local farmers, since it’s one of the world’s best fertilizers), which is then eaten and condensed by six species of flesh-eating beetles (a type of insect that natural-history museums and medical schools use to clean bones). The guano and bugs release so much ammonia into the cave that if a human entered and remained for more than thirty minutes, she would die. Hutchins has entered the cave dozens of times to collect data, wearing a full respirator. As the sun came up, clouds of bats seemed to appear out of nowhere; they arrived from great heights and from every direction, so that it was almost impossible to see them until they were whizzing by my ear, or directly in front of our eyes. As the light increased and the bats became more vulnerable to prey, especially hawks, they stopped circling, folded up their wings, and let gravity take hold. They dive-bombed the cave mouth, dropped in heavy heaps, found their babies inside. After an hour, the sun was up and it was time to go. Bats continued to rain from the sky, their mystery intact, as we got in our cars and drove to Denny’s.

Andrea Turkalo, a researcher who has spent more than twenty years camped
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